1. Technical Field
This invention relates to a piston ring for an internal combustion engine, and more particularly, to such a piston ring formed of a fiber reinforced ceramic matrix composite (FRCMC) material and methods for making it.
2. Background Art
In the basic operation of a gasoline-powered internal combustion engine, a piston moving up and down within a cylinder draws in a combustible mixture of fuel and air on its down-stroke and compresses it on its up-stroke. The compressed mixture is then ignited and burns and expands driving the piston down. A similar process occurs in other types of engines. Piston rings are designed primarily to seal the combustion chamber area of the cylinder. If the piston is not properly sealed within the cylinder, blow-by occurs and the compression of the combustible mixture is reduced or non-existent thereby reducing or eliminating the effectiveness of that piston/cylinder within the engine. Additionally, on combustion of the mixture, piston rings prevent blow-by of the expanding gasses between the piston and the wall of the cylinder. Piston rings are installed near the top of the piston. The ring must exert even pressure against the cylinder wall for the full circumference of the cylinder wall to prevent blow-by and ensure engine efficiency. Therefore, piston rings are designed so that the diameter before installation is slightly larger than that of the cylinder bore. As a result, piston rings are exposed to great stresses during the process of mounting them on the piston head and tend to fracture during the installation process. The piston and piston rings are exposed to high pressures and high pressure variations, Additionally, the piston rings are exposed to frictional heating as well as heating due to the combustion of the fuel mixture. Hence, they are exposed to high temperatures and temperature variations, making them prone to thermally-induced failure. While the principles of the present invention are applicable to a wide variety of engine types, a standard cylindrical piston moving up and down within a cylindrical cylinder of an engine block having one or more cylinders will be used as the embodiment chosen for description hereinafter and in the drawings which accompany it. It should be understood, however, that it is intended that the invention described and claimed herein be accorded a breadth in keeping with the scope and spirit of the disclosure as applied to all engine types.
In a classic engine, the cylinders and pistons are made of metal. Cast iron is the conventional material used for piston rings, though aluminum, steel and other materials are used. Conventional piston rings are often chrome plated or molybdenum coated or coated with other materials to provide increased life and greater freedom from scoring.
As stated previously, the standard method of sealing the space between the cylinder walls and the piston is the piston ring. A space exists (and must exist) between the piston and the cylinder because of the dissimilar thermal expansion of the metal components. If the piston and cylinder were sized exactly with only enough clearance for the piston to fit within the cylinder with a coating of lubricating oil film between them, as soon as the metal heated from the combustion within the cylinders and expanded, the piston would seize within the cylinder. If enough clearance was provided initially to allow for expansion, the blow-by would be so extreme that the engine would not run sufficiently to get up to temperature and create the proper seal. Thus, the expansion space is provided and the clearance gap is closed with piston rings as shown in FIGS. 1 and 2. There may be one piston ring, two piston rings, or more, depending on the engine design and the objectives thereof. Regardless of the number, each piston ring 10 is disposed in a ring groove 16 in the peripheral surface of the piston 14 adjacent to the top thereof. The piston ring 10 is not a complete circle. Rather, it is incomplete and has a gap 12 at its ends so that it can be compressed or expanded in diameter within the ring groove 16. Typically, a piston ring 10 operates by expanding so that its outer peripheral surface presses against the cylinder wall to seal the expansion space between the piston 14. This expansion is caused by the high pressure gases formed in the cylinder above the piston 14. These gases flow down the between the piston and cylinder wall. These high pressure gases first force the piston ring 10 downward in the piston ring groove 16. The gases then enter the ring groove 16 from a point above the piston ring 10, and eventually flow behind the piston ring where they force it outwards to seal the expansion space. The piston ring 10 is hence exposed to frictional forces due to its movement within the ring groove 16 and up and down within the cylinder, which decrease the life of the ring due to friction-induced wear. Hardness and lubricity of the rings are hence a desirable quality. Hardness limits the amount of wear, such as scoring, due to friction. Lubricity between the parts is essential to provide proper operation of the piston/cylinders and to prevent seizure of these components and also reduce friction between the piston, piston ring and cylinder components.
Early low compression engines with piston rings made of the materials available at the time tended to form carbon within the ring grooves over time. Recent engine designs and the materials employed for the pistons, cylinders, and rings as well as the modern more accurately computer-controlled engines have resulted in less tendency for there to be formation of carbon in the ring grooves which caused the piston ring to stick within the piston groove.
While engine designs and materials have certainly improved over the years, there still remain deficiencies such as lower than desirable fuel efficiency and higher than desirable pollution emissions. In a co-pending application entitled HIGH-EFFICIENCY, LOW-POLLUTION ENGINE by the inventors of this application and assigned to the common assignee, an improved fiber reinforced ceramic matrix composite (FRCMC) material is disclosed having high breakage resistance and particular applicability to use for parts in a high temperature internal combustion engine. This copending application was filed on Aug. 16, 1995 and assigned Ser. No. 08/515,604. The disclosure of the co-pending application is incorporated by reference. The co-pending application taught FRCMC pistons and cylinders. Being of the FRCMC material, the pistons and cylinders can withstand much higher operating temperatures than conventional internal combustion engines. Operation at higher temperatures increases fuel efficiency and reduces the pollution produced because fuel and contaminants are more completely burned at higher temperatures. Moreover, since the coefficient of thermal expansion of the parts is much lower, decreasing the allowance required for the dissimilar thermal expansion of the components, much closer tolerances can be maintained without the danger of engine seizure. Still, however, to operate at as high a compression ratio as possible without efficiency-robbing blow-by, there must be piston rings. Conventional piston rings are not adequate, however. Despite the many improvements in materials and wear resistance, prior art piston rings are intended for use in metal engines. The FRCMC material of the pistons and cylinders, in conjunction with its ceramic wear coatings will quickly erode even the hardest conventional metallic piston rings.
To prevent the common piston ring problems, car, motorcycle, truck, train, and other machinery applications could utilize a better piston ring than is provided by current technology, especially those employing FRCMC pistons and/or cylinders. Depending on the application, this improved piston ring should be constructed of material that is light, long wearing, and which has a low thermal expansion so as to allow manufacture of the engine parts to close tolerances. This ensures a longer life and improved performance over the present technology. Additionally, this improved piston ring should be hard to endure the stresses of piston/cylinder operation and be tolerant of high temperatures to which they will be exposed as a result of the combustion within the cylinders and the friction of the interfacing parts. It should also be made of material that has a low coefficient of friction so as to reduce the wear due to the friction of the various piston components rubbing together during piston/cylinder operation.
Wherefore, it is an object of this invention to provide a lightweight, but high strength piston ring which is ductile and fracture resistant.
It is another object of the present invention to provide a piston ring that can be used in a FRCMC cylinder or in combination with a FRCMC piston without rapid erosion.
It is still another object of this invention to provide a piston ring that has a lower thermal expansion, so as to allow engine parts to be machined to much closer tolerances without the danger of engine seizure.
It is still another object of this invention to provide a piston ring that exhibits high hardness and good lubricity at elevated temperatures.
Wherefore, it is another object of the present invention to provide a piston ring which is capable of withstanding high temperatures and thermally-induced strains.
Other objects and benefits of this invention will become apparent from the description which follows hereinafter when read in conjunction with the draw figures which accompany it.